fix(core): register tom_thumb, accept frame_hold in the test double, wire api_v3's managers

Three independent fixes found while validating every plugin on a 256x64 rig.

FontManager never registered tom_thumb even though assets/fonts/tom-thumb.bdf
ships with the core, so every plugin offering it logged "Font family
'tom_thumb' not found" (16 warnings per countdown render) and had to carry a
private loader to use a bundled font. Closes #524.

VisualTestDisplayManager.set_scrolling_state() lacked the frame_hold parameter
that DisplayManager gained, so any plugin passing it died with TypeError at
render time and failed every size. Nine plugins now make that call;
ledmatrix-stocks and ledmatrix-leaderboard were failing outright and the other
seven only passed because their scroll path was unreachable without data.
Closes #525.

api_v3 declared module-level config_manager/plugin_manager = None that nothing
ever assigned -- app.py sets the blueprint attributes, which the other 150+
call sites use. Three sites read the decoys, so /health reported the config
unreadable and the plugin system uninitialised (making "degraded" permanent and
unreachable-by-design) and /display/current fell back to a hardcoded 128x64 on
every rig. The decoys are removed rather than assigned, so a bare name is now a
NameError at test time instead of a silent None. The same function's first-call
uptime was computed from two separate clock reads and came out negative.
Closes #529.

Verified on the rig: both previously-failing plugins render, the tom_thumb
warnings are gone, /health reports "healthy" with all three checks passing, and
/display/current reports the real 256x64.

src/font_manager.py

@@ -96,7 +96,8 @@ class FontManager:
         self.common_fonts = {
             "press_start": "assets/fonts/PressStart2P-Regular.ttf",
             "four_by_six": "assets/fonts/4x6-font.ttf",
-            "five_by_seven": "assets/fonts/5x7.bdf"
+            "five_by_seven": "assets/fonts/5x7.bdf",
+            "tom_thumb": "assets/fonts/tom-thumb.bdf"
             # Note: cozette_bdf removed - font file not available
             # To re-enable: download cozette.bdf from https://github.com/the-moonwitch/Cozette
             # and add: "cozette_bdf": "assets/fonts/cozette.bdf"

src/plugin_system/testing/visual_display_manager.py

@@ -506,9 +506,16 @@ class VisualTestDisplayManager:
     # Scrolling state (no-op interface compat)
     # ------------------------------------------------------------------
 
-    def set_scrolling_state(self, is_scrolling: bool):
-        """Set the current scrolling state (no-op for testing)."""
+    def set_scrolling_state(self, is_scrolling: bool, frame_hold: int = 1):
+        """Set the current scrolling state (no-op for testing).
+
+        ``frame_hold`` mirrors DisplayManager.set_scrolling_state so a plugin
+        that paces its scroll can be rendered here. Without it every such
+        plugin raised TypeError at render time and failed every size, which is
+        invisible until a plugin happens to pass the argument.
+        """
         self._scrolling_state['is_scrolling'] = is_scrolling
+        self._scrolling_state['frame_hold'] = frame_hold
         if is_scrolling:
             self._scrolling_state['last_scroll_activity'] = time.time()